1. Field of the Invention
The present invention relates to a coil-form electrolytic capacitor that is used in various electronic appliances.
2. Description of the Related Art
With the recent tendency toward popularization of high-frequency electronic appliances, high-performance electrolytic capacitors are needed that have a reduced equivalent series resistance (ERS) in a high-frequency region for the parts of such electronic appliances. For reducing their ERS in a high-frequency region, solid electrolytic capacitors are investigated these days that comprise a solid electrolyte such as an electroconductive polymer having a high electric conductivity. To satisfy the requirement for increasing their capacity, solid electrolytic capacitors are being put on the market that are constructed by coiling up an anode foil and a cathode foil with a separator sandwiched therebetween followed by infiltrating an electroconductive polymer into the thus-constructed coil-form capacitor device.
In the coil-form electrolytic capacitor, a separator is an indispensable element to be between the anode foil and the cathode foil for preventing them from being in contact with each other. For the separator, for example, employed is ordinary electrolytic paper formed of Manila hemp or kraft paper as in ordinary electrolytic capacitors that contain an ordinary driving electrolytic solution as the electrolyte therein. The coiled capacitor device having such electrolytic paper for separator is heated so as to carbonize the electrolytic paper (thus carbonized, the paper is hereinafter referred to as carbonized paper). Apart from it, a nonwoven fabric of glass fibers or a nonwoven fabric of synthetic fibers such as vinyl on, polyester or polyamide may also be used for the main ingredient of the separator.
For the electroconductive polymer to be used for the solid electrolyte, known is poly-3,4-ethylenedioxy-thiophene prepared through chemical oxidation polymerization of a cationic component and an anionic component where the cationic component functions as an oxidizing agent owing to the reduction of the metal ion and the anionic component serves as a dopant and therefore functions both as an oxidizing agent and a dopant, such as typically polymerization of 3,4-ethylenedioxy-thiophene with ferric p-toluenesulfonate. Also known for it is polypyrrole prepared through chemical oxidation polymerization of a monomer pyrrole with ferric chloride or persulfate that serves as an oxidizing agent and also as a dopant.
On the other hand, coil-form electrolytic capacitors are also proposed, in which both a solid electrolyte of electroconductive polymer and a driving electrolytic solution are used for the material of cathode lead.
The coil-form electrolytic capacitors of the type proposed include, for example, the following: A separator formed of Manila paper or kraft paper, or a separator formed of a porous film or a nonwoven fabric of synthetic fibers is processed with an electroconductive polymer that is prepared through chemical oxidation polymerization with a persulfate serving both as an oxidizing agent and as a dopant, whereby the separator is made electroconductive, and the resulting electroconductive separator is used along with a driving electrolytic solution to construct an electrolytic capacitor (for example, JP-A-1-1090517 and JP-A 7-249543). A coil-type capacitor device is infiltrated with an electroconductive polymer and a driving electrolytic solution to construct an electrolytic capacitor (for example, JP-A 11-186110).
In the above-mentioned coil-form solid electrolytic capacitor, however, a solid electrolyte of an electroconductive polymer or the like that could poorly repair dielectric oxide film is used. Therefore, it is difficult to construct voltage-proof capacitors, and only capacitors having a rated voltage of at most 25 to 32 V or so could be obtained.
Even within the rated voltage range, the capacitors may have a trouble of short circuit owing to unexpected increase in leak current during their use or owing to the formation of defects in the dielectric oxide film, and therefore the capacitors must be used after their trouble rate is explicitly computed. However, this is troublesome in using the capacitors.
In addition, during the process of producing the solid electrolytic capacitors, they may frequently have a trouble of short circuit during aging, and the percent defective in their production is extremely high as compared with that in producing electrolytic capacitors that comprise a driving electrolytic solution. This is another problem with the solid electrolytic capacitors.
The trouble of short circuit could be overcome in some degree by using a high-density separator or a heat-resistant separator (e.g., separators consisting essentially of polyester resin or aramid resin), but the coil-form solid electrolytic capacitors are still unsatisfactory in this respect, as compared with conventional electrolytic capacitors that comprise a driving electrolytic solution alone.
For overcoming the trouble, electrolytic capacitors are proposed in which both a solid electrolyte of electroconductive polymer and a driving electrolytic solution are used for the material of cathode lead. In these, however, since the solid electrolyte is formed of an electroconductive polymer prepared through chemical oxidation polymerization with an inorganic acid salt, persulfate such as sodium persulfate or ammonium persulfate that serves both as an oxidizing agent and as a dopant, the persulfate ion that functions as the dopant is readily dedoped to dissolve out in the driving electrolytic solution, and the electroconductivity of the electroconductive polymer is significantly lowered owing to the dedoping. Therefore, the solid electrolyte has some problems in that its thermal stability in electrolytic capacitors is poor and its ESR greatly varies with time in a high-frequency region.
A method for producing a solid electrolytic capacitor is known, which comprises chemical oxidation polymerization of 3,4-ethylenedioxy-thiophene with a hardly-dedopable transition metal-based oxidizing agent such as ferric p-toluenesulfonate that has the ability to assist oxidation-reduction of metal ion and serves also as a dopant. When the capacitor device having the constitution is infiltrated with a driving electrolytic solution to construct an electrolytic capacitor, then the remaining metal ion component may dissolve out and deposit in the driving electrolytic solution owing to the electrochemical reaction thereof in the solution, and, as a result, the leak current from the electrolytic capacitor may increase. This is one problem with the electrolytic capacitor of the type. Even when the solid electrolytic capacitor is constructed not infiltrated with a driving electrolytic solution, it could not still solve the problem. This is because, when the solid electrolytic capacitor is used in a high-humidity atmosphere, then the remaining metal ion component will also dissolve out and deposit in it owing to the electrochemical reaction of the metal ion in water that may penetrate into it through the sealed opening of the capacitor and, as a result, the leak current from the capacitor may also increase.